1. Field of the Invention
The present invention relates to a method for transmission (the definition of which is intended to include a term of "recording") of variable length coded data with less affection of transmission error and also, to a video signal coding apparatus for production of code data of a given transmission format which are encoded at high efficiency through variable length coding and can be decoded with a common decoder designed for processing conventional video signals which are different from the video signal of the apparatus.
2. Description of the Prior Art
Variable length coding, e.g. Huffman coding, has widely been employed for achievement of highly efficient data transmission, in which short code words are assigned to particular data which are frequently used so that the overall amount of code elements to be transmitted can be minimized.
In general, variable length code words are allocated to data storage fields in the transmission format and after accompanied with an error correcting code, modulated for transmission. The variable length code words in the data storage field are aligned from the front end in the bit serial arrangement.
FIG. 1 illustrates a group of variable length code words allocated to a given area for transmission in a prior art. More specifically, nine variable length code words C1 to C9 are allocated to a data storage field which has a 23-bit length (smaller than a common size for ease of description).
The hatching area in FIG. 1 represents a vacant space where no variable length code word is loaded. As the allocation of variable length code words to the data storage field substantially leaves such a gap space, extra information is needed for indicating the end of a stream of the storage data.
For application of such information, there are some methods including a technique of adding a particular code (referred to as a mark code) to the end of the stream of the variable length code words stored in the data storage field and a technique of separately transmitting the information about the size of the variable length code word stream as side information. Although any method is eligible with the present invention, the description will be proceeded with respect to the technique using a mark code. The mark code is represented by C9 in FIG. 1 as exhibiting in the end of the variable length code word stream.
As each variable length code word is different in length, an error, when occurs, will be propagated. As the result, the variable length code words after the location of the error will be no more decoded correctly.
The disadvantage of the prior art technique of transmission of variable length code words which are simply aligned in one row is that when an error occurs in a data storage field, the variable length code words after the error location in the storage field will be no more decoded correctly (see "An experimental study for a home-use digital VTR" by C. Yamamitsu et al, IEEE Trans. CE, CE-35, No. 3, Aug. 1989, pp 450-457). The error location is denoted by the mark X in FIG. 1 and the remaining of the variable length code words which cannot be decoded correctly is indicated by the real line arrow.
It is known that for ease of recording or transmission of video signals which carry a large amount of data, the data are compressed by a high efficiency coding so that no noticeable failure can appear in reproduced pictures. Such a high efficiency coding commonly involves orthogonal transformation and variable length coding.
There have been developed different TV systems, e.g. EDTV and high definition TV of which aspect ratio is 16:9 as compared with 4:3 in the conventional TV system. Accordingly, an improved coding apparatus is required for processing different type video signals. However, a known coding apparatus which is designed for processing video signals associated with a 4:3 aspect ratio cannot handle specific coded data produced from 16:9 aspect ratio video signals by means of the high efficiency coding.